Fix load balancing bug in streaming replication mode.

Fix load balancing bug in streaming replication mode.

In an explicit transaction, any SELECT will be load balanced until
write query is sent. After writing query is sent, any SELECT should be
sent to the primary node. However if a SELECT is sent before a sync
message is sent, this does not work since the treatment of writing
query is done after ready for query message arrives. Solution is, the
treatment for writing query is done in executing the writing query as
well.

The bug has been there since V3.5.